This invention is directed to a process of adjusting a positioning device including a scanner which scans markers and a circuit for carrying out the process. Positioning devices are used in record players having record changers, and in jukeboxes to position the playing element in front of the magazine in which the individual records are stored. The device which transports the record to the playing element must be accurately positioned in the proximity of the desired record.
A compact disc changer having a magazine and a scanning device is described in WO-A-88/09033. The scanning device includes an optical transmitter to emit a laser beam, to scan the compartments of a magazine. The light beam is reflected from a compact disc lying in one of the compartments of the magazine onto a photodetector, the output of which is connected to a control circuit. The scanning device, which is mounted on a shaft driven by a motor, can be positioned in front of any compartment of the magazine. In order to precisely position the scanning device at the level of each individual magazine compartment, a disc with markers is mounted on one end of the shaft, and the disc is scanned by a second scanning device which serves as an incremental transmitter.
Patent GB-A 2 162 991 discloses a record changer having a ledge including slots, which serve as markers, positioned in front of the record magazine. The ledge is arranged between the optical transmitter and the optical detector of the light. The playing element is accurately positioned in front of a record by detecting the individual slots within the ledge. When the playing element passes a slot, the output voltage of the photo detector is approximately sine shaped, as shown in FIG. 1. The output of the photo detector is converted to a pulse, the rising or falling edge of which can be used to indicate that the playing element is positioned in front of a record.
In order to ensure exact positioning of the scanning device during normal operation, the scanning device must be precisely adjusted manually during production of the record changer. Accordingly, the scanning device is initially precisely positioned manually using a gauge and is then fixed into position. Subsequently the photodetector is also manually adjusted so that the output signal of the photodetector is maximized. These critical but essential adjustments are time consuming and expensive. The invention provides a simplified method of adjusting a positioning device that is quick and uncomplicated.
With the invention, the scanning device is precisely set at a defined position. The scanning device is then automatically moved until the output signal from the photodetector goes to maximum. The deviation between the first position and the second position is stored as an offset. During normal operation the scanning device is initially positioned at the position where the output signal of the detector is maximum. The scanning device then is moved by the stored offset to the final position. In another adjustment technique the sensor output is converted into a pulse and the pulse width is measured. The scanning device is then manually positioned at a defined position after which the scanning device is automatically positioned in a second position which is shifted by half of the pulse width from the first position.